nixos/buildkite: drop user option (#78160)

* nixos/buildkite: drop user option

This reverts 8c6b1c3e.

Turns out, buildkite-agent has logic to write .ssh/known_hosts files and
only really works when $HOME and the user homedir are in sync.

On top of that, we provision ssh keys in /var/lib/buildkite-agent, which
doesn't work if that other users' homedir points elsewhere (we can cheat
by setting $HOME, but then getent and $HOME provide conflicting
results).

So after all, it's better to only run the system-wide buildkite agent as
the "buildkite-agent" user only - if one wants to run buildkite as
different users, systemd user services might be a better fit.

* nixosTests.buildkite-agent: add node with separate user and no ssh key